Bimetallic Borohydrides in the System M(BH4)2–KBH4 (M = Mg, Mn): On the Structural Diversity

Four novel bimetallic borohydrides have been discovered, K2M(BH4)4 (M = Mg or Mn), K3Mg­(BH4)5, and KMn­(BH4)3, and are carefully investigated structurally as well as regarding their decomposition reaction mechanism by means of in situ synchrotron radiation powder X-ray diffraction (SR-PXD), vibrational spectroscopies (Raman and IR), thermal analysis (TGA and DTA), and ab initio density functional theory (DFT) calculations. Mechano-chemical synthesis (ball-milling) using the reactants KBH4, α-Mg­(BH4)2, and α-Mn­(BH4)2 ensures chlorine-free reaction products. A detailed structural analysis reveals significant similarities as well as surprising differences among the two isomorphs K2M(BH4)4, most importantly concerning the extent to which the complex anion [M(BH4)4]2– is isolated in the structure. Anisotropic thermal expansion and an increase in symmetry at high temperatures in K3Mg­(BH4)5 is ascribed to the motion of BH4 groups inducing hydrogen repulsive effects, and the dynamics of K3Mg­(BH4)5 are investigated. Decomposition in the manganese system proceeds via the formation of KMn­(BH4)3, the first perovkite type borohydride reported to date.